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close this bookBiogas Plants in Animal Husbandry (GTZ, 1989)
close this folder5. Biogas technique
View the document(introduction...)
View the document5.1 Fundamental principles, parameters, terms
View the document5.2 Design principles of simple biogas plants
View the document5.3 Biogas plants of simple design
View the document5.4 Design and construction of plant components
View the document5.5 Biogas utilization
View the document5.6 Measuring methods and devices for biogas plants

5.5 Biogas utilization

5.5.1 Composition and properties of biogas
5.5.2 Conditioning of biogas
5.5.3 Biogas appliances
5.5.4 Biogas-fueled engines

5.5.1 Composition and properties of biogas

Biogas is a mixture of gases that is composed chiefly of:

- methane, CH4

40 - 70 vol. %

- carbon dioxide, CO2

30-60 vol. %

- other gases

1 - 5 vol.%, including

- hydrogen H2

0-1 vol. %

- hydrogen sulfide, H2S

0-3 vol. %

Like those of any gas, the characteristic values of biogas are pressure and temperature-dependent. They are also affected by water vapor. The factors of main interest are:

- volumetric change as a function of temperature and pressure,
- change in value as a function of temperature, pressure and water-vapor con" tent, and
- change in water-vapor content as a function of temperature and pressure.
Chapter 10.2 contains pertinent tables, formulae and nomograms for use in calculating conditions of state.

5.5.2 Conditioning of biogas

While the biogas produced by the plant can normally be used as it is, i.e. without further treatment/conditioning, various conditioning processes are described in this chapter to cover possible eventualities.

Reducing the moisture content of the biogas, which is usually fully saturated with water vapor. This involves cooling the gas, e.g. by routing it through an underground pipe, so that the excess water vapor condenses out at the lower temperature. When the gas warms up again, its relative vapor content decreases (cf. chapter 10.2 for calculations). The "drying" of biogas is especially useful in connection with the use of dry gas meters, which otherwise would eventually fill up with condensed water.

Table 5.14: Composition and properties of biogas, and its constituents under s.t.p. conditions (0 °C, 1013 mbar)
(Source: OEKOTOP, compiled from various sources)

Constituents and properties

CH4

CO2

H2

H2S

60% CH4/
40% CO2

65% CH4/
34% C02/
1% rest

Volume fraction (%)

55-70

27-44

1

3

100

100

Net calorific value (kWh/m³)

9.9

-

3.0

6.3

6.0

6.8

Ignition threshold (% in air)

5-1S

-

4-80

4-45

6-12

7.7 - 23

Ignition temperature (°C)

650-750

-

585

-

650-750

650-750

Crit.pressure (bar)

47

75

13

89

75-89

75-89

Crit. temp. (°C)

-82.5

31.0

-240

100.0

-82.5

-82.5

Normal density (g/1)

0.72

1.98

0.09

1.54

1.2

1.15

Gas/air-density ratio

0.55

2.5

0.07

1.2

0.83

0.91

Wobbe index, K (kWh/m³)

13.4

-

-

-

6.59

7.15

Spec. heat, cp (kI/m³ °C)

1.6

1.6

1.3

1.4

1.6

1.6

Flame propagation (cm/s)

43

-

47

-

36

38

Reduction of the hydrogen-sulfide content (H2S) may be necessary if the biogas is found to contain an excessive amount, i.e. more than 2%, and is to be used for fueling an engine. Since, however, most biogas contains less than 1% H2S, desulfurization is normally unnecessary, especially if it is to be used for operating a stationary engine.

For small-to-midsize systems, desulfurization can be effected by absorption onto ferric hydrate (Fe (OEI)3), also referred to as bog iron, a porous form of limonite. The porous, granular purifying mass can be regenerated by exposure to air.

The absorptive capacity of the purifying mass depends on its iron-hydrate content: bog iron containing 5-10% Fe(OH)3 can absorb about 15 g sulfur per kg without being regenerated and approximately 150 g/ kg through repetitive regeneration. It is a very noteworthy fact that many types of tropical soil (laterites) are naturally ferriferous and, hence, suitable for use as purifying mass.

Reduction of the carbon-dioxide content (CO2) is very complicated and expensive. In principle, CO2 can be removed by absorption onto lime milk, but that practice produces "seas" of lime paste and must therefore be ruled out, particularly in connection with large-scale plants, for which only high-tech processes like microscreening are worthy of consideration. CO2 "scrubbing" is rarely advisable, except in order to increase the individual bottling capacity for high-pressure storage.


Fig. 5.29: Ferric-hydrate gas purifier. 1 Gas pipe, 11 Raw-gas feed pipe, 12 Clean-gas discharge pipe, 13 Purging line, 2 Metal gas purifier, 3 Shelves for purifying mass, 4 Purifying mass (Source: Muche 1984)

Table 5.15: Pointers on flame adjustment (Source: OEKOTOP)

Problem

Cause - Remedy

elongated, yellow- ish flame

lack of combustion air - open the air supply

flame "lifts off"

excessive exit velocity - use smaller injector, reduce the gas pressure, reduce the air supply

flame "flashes back"

exit velocity too low - use larger injector, increase the gas pressure, open the air supply, reduce the size of the burner jets

flame "too small"; not enough fuel

fuel shortage - use larger injector, increase the gas pressure

flame "too big"; excessive fuel supply/consumption

excessive fuel supply - reduce the gas pressure, use smaller injector


5.5.3 Biogas appliances

Biogas is a lean gas that can, in principle, be used like any other fuel gas for household and industrial purposes, the main prerequisite being the availability of specially designed biogas burners or modified consumer appliances. The relatively large differences in gas quality from different plants, and even from one and the same plant (gas pressure, temperature, calorific value, etc.) must be given due consideration.

The heart of any gas appliance is the burner. In most cases, atmospheric-type burners operating on premixed air/gas fuel are considered preferable.

Due to complex conditions of flow and reaction kinetics, gas burners defy precise calculation, so that the final design and adjustments must be arrived at experimentally.


Fig. 5.30: Schematic drawing of a biogas burner and its parts. 1 Gas pipe, 2 Gas-flow shutoff/reducing valve, 3 Jets (f = 1-2 mm), 4 Mixing chamber for gas and combustion air, 5 Combustion air intake control, 6 Burner head, 7 Injector (Source: Sasse 1984)

Accordingly, the modification and adaptation of commercial-type burners is an experimental matter. With regard to butane and propane burners, i.e. the most readily available types, the following pointers are offered:

- Butane/propane gas has up to 3 times the calorific value of biogas and almost twice its flame-propagation rate.

- Conversion to biogas always results in lower performance values.

Practical modification measures include:

- expanding the injector cross section by a factor of 2-4 in order to increase the flow of gas

- modifying the combustion-air supply, particularly if a combustion-air controller is provided - increasing the size of the jet openings (avoid if possible) The aim of all such measures is to obtain a stable, compact, slightly bluish flame.

Table 5.16: Comparison of various internationally marketed biogas burners (Source: OEKOTOP, compiled. from various sources)

Type of burner¹

Number of flames

Gas consumption

Burning properties

Handling

Peking No. 4/PR China (3)

1

200 l/h

+

o

Jackwal/Brazil (1)

2

2 X 1501/h

++

+

Patel GC 32/ludia

2

2 X 2501/h

o

++

Patel GC 8/India

1

2301/h

+

++

KIE burner/Kenya (2)

2

?

+

++

++ very good +good o average
1 Number of burner shown in figure 5.31


Fig. 5.31: Various types of biogas burners. 1 2-flame lightweight burner (2 X 1501/h), 2 2-flame stable burner (2 X 2501/h), 31-flame burner (200 I/h) (Source: OEKOTOP)

Gas cookers/stoves

Biogas cookers and stoves must meet various basic requirements:
- simple and easy operation
- versatility, e.g. for pots of various size, for cooking and broiling
- easy to clean
- acceptable cost and easy repair
- good burning properties, i.e. stable flame, high efficiency
- attractive appearance

A cooker is more than just a burner. It must satisfy certain aesthetic and utility requirements, which can vary widely from region to region. Thus, there is no such thing as an all round biogas burner.
Field data shows that 2-flame stable burners are the most popular type (cf. fig. 5.31).

Table 5.17: Biogas consumption for cooking (Source: OEKOTOP, compiled from various sources)

To be cooked:

Gas consumption

Time

11 water

30-40 l

8-12 min

51 water

110-140 l

30-40 min

31 broth

~60 l/h


1/2 kg rice

120-140 l

~40 min

1/2 kg legumes

160-190 l

~60 min

1 tortilla(fried)

10-20 l

~3 min

Gas consumption per person and meal

150-300 l/d


Gas consumption per 5-member family

1500 -2400 l/d


(2 cooked meals)



Single-flame burners and lightweight cookstoves tend to be regarded as stop-gap solutions for want of suitable alternatives.

Biogas cookers require purposive installation with adequate protection from the wind. Before any cooker is used, the burner must be carefully adjusted, i.e.:

- for a compact, bluish flame,
- the pot should be cupped by the outer cone of the flame without being touched by the inner cone,
- the flame should be self-stabilizing, i.e. flameless zones must re-ignite automatically within 2 to 3 seconds.

Test measurements should be performed to optimize the burner setting and minimize consumption. The physical efficiency of a typical gas burner ranges from 0.6 to 0.8.

Table 5.18: Tests for biogas cookers/stoves (Source: OEKOTOP)

1. Measuring the efficiency with water


h =- burner efficiency ( - )
QW = quantity of heated water (kg)
T1,T2 = initial and final temperature (°C)
cW = spec. heat capacity = 4.2 kJ/kg
EW = quantity of evaporated water (kg)
L = evaporation heat loss = 2260 kJ/kg
n.c.v. = net cal. value of biogas (kJ/m3 )
Q = quantity of biogas (m3)
——————————————————————————————————————————
2. Gas consumption for holding the temperature at boiling point (simmering temperature -95 °C), i.e. the amount of gas needed per unit of time to maintain a water temperature of 95 °C
——————————————————————————————————————————
3. Standard cooking test
This test determines how much gas is- needed to cook a standard meal, e.g. 500 g rice and 1000 g water; the standard meal is specified according to the regional staple diet
——————————————————————————————————————————
4. Complete-meal tests
Everything belonging to a complete meal is cooked by a native person.
——————————————————————————————————————————


Fig. 5.32: Schematic drawing of a biogas lamp. 1 Gas pipe, 21 Shutoff valve, 22 Adjusting valve, 3 Primary air supply (adjustable), 4 Mixing chamber, 5 Incandescent body - gas mantle, 6 Porcelain head, 7 Disk reflector, 8 Glass (Source: OEKOTOP/ Jackwal)

Biogas lamps

The bright light given off by a biogas lamp is the result of incandescence, i.e. the intense heat-induced luminosity of special metals, so-called "rare earths" like thorium, cerium, lanthanum, etc. at temperature of 1000 - 2000 °C.

At 400-500 lm, the maximum light-flux values that can be achieved with biogas lamps are comparable to those of a normal 25-75 W light bulb. Their luminous efficiency ranges from 1.2 to 2 Im/W. By comparison, the overall efficiency of a light bulb comes to 3-5 Im/W, and that of a fluorescent lamp ranges from 10 to 15 lm/W.

The performance of a biogas lamp is depenent on optimal tuning of the incandescent body (gas mantle) and the shape of the flame at the nozzle, i.e. the incandescent body must be surrounded by the inner (= hottest) core of the flame at the minimum gas consumption rate. If the incandescent body is too large, it wil1 show dark spots; if the flame is too large, gas consumption will be too high for the light-flux yield. The lampshade reflects the light downward, and the glass prevents the overly rapid loss of heat.

Table 5.19: Standard lighting terms and units of measure (Source: OEKOTOP)

Term/definition

Unit, formula

Luminous flux (F)

F, measured in lm (lumen)

The light output defined as the luminous flux of a black body at 2042 °K per cm²


Luminous intensity (I)

I, measured in cd (candela)

The solid-angle light power


I = luminous flux / solid angle (w)

I = F/w (cd = lm/w)


half-space w = 2 p = 6.28

Illuminance (E)

-E, measured in lux (Ix)

light power per unit area


E = luminous flux / area (A)

E = F/A (lx = lm/m²)

Spec. illuminance (Es)

Es = ((E x r²) / V · n.c.v.)) · (lx · m² / kW)

Effective incident illuminance, as measured normal to the light source at a defined distance from the source referred to the input

E = meas. illuminance


r = distance between the incandescent body and the photoelectric cell


V = biogas consumption n.c.v. = net calorific value

Luminous efficiency (Re) light power referred to the energy input (Ei)

Re = F/Ei (lm/kW)


Sample calculation

Measured values:

Results:

Illuminance

Luminous intensity

E=901x

I = E x r² = 90 cd

meas. distance, r = 1.0 m

luminous flux

gas consumption, V = 110 1/h

F = I x w = 90 x 6.28 = 565 lm

cal. value, n.c.v. = 6 kWh/m³

luminous efficiency


Re = F:Q = 565:110 = 5.1 lm/lxh


Re = F/Ei = 565:660 = 0.9 lm/W

Practical experience shows that commercial type biogas lamps are not optimally designed for the specific conditions of biogas combustion (fluctuating or low pressure, varying gas composition). The most frequently observed shortcomings are:

- excessively large nozzle cross sections
- excessively large gas mantles
- no possibility of changing the injector
- poor or lacking means of combustion-air control.

Such drawbacks result in unnecessarily high gas consumption and poor lighting. While the expert/extension officer has practically no influence on how a givenlamp is designed, he can at least give due consideration to the aforementioned aspects when it comes to selecting a particular model.

Table 5.20: Comparison of various biogas lamps (Source: Biogas Extension Program)

Type of lamp

Suitability1

Gas consumption

D 80 - 3 Juojiang/PR China

o 2

?

Avandela - Jackwal/Brazil

+

100 l/h

Patel Outdoor-single/India

++

150 l/h

Camping-Gas

+

?

1 Quality criteria: gas consumption, brightness, control
2 Quality ratings: ++ very good, + good, o average

Biogas lamps are controlled by adjusting the supply of gas and primary air. The aim is to make the gas mantle burn with uniform brightness and a steady, sputtering murmer (sound of burning, flowing biogas). To check the criteria, place the glass on the lamp and wait 2 - 5 minutes, until the lamp has reached its normal operating temperature. The lamps compared in table 5.20 operate at a gas pressure of 5 - 15 cmWG. If the pressure is any lower, the mantle will not glow, and if the pressure is too high (fixed-dome systems) the mantle may tear.

Adjusting a biogas lamp requires two consecutive steps:

1. precontrol of the supply of biogas and primary air without the mantle, initially resulting in an elongated flame with a long inner core;

2. fine adjustment with the incandescent body in place, resulting in a brightly glowing incandescent body, coupled with slight further adjustment of the air supply (usually more).

The adjustment is at its best when the dark portions of the incandescent body have just disappeared. A luxmeter can be used for objective control of the lamp adjustment.


Fig. 5.33: Schematic drawing of a radiant heater. 1 Gas pipe, 2 Shutoff valve, 3 Safety pilot, 31 Heat sensor, 4 Mixing chamber, 5 Air supply, 6 Injector, 7 Ceramic panel with protective screen, 8 Reflector, 9 Hanger (Source: OEKOTOP / SBM)

Radiant heaters

Infrared heaters are used in agriculture for achieving the. temperatures required for raising young stock, e.g. piglets and chicks, in a limited amount of space. The nursery temperature for piglets begins at 30-35 °C for the first week and than gradually drops off to an ambient temperature of 18-23 °C in the 4th/5th week. As a rule, temperature control consists of raising or lowering the heater. Good ventilation is important in the stable/nursery in order to avoid excessive concentrations of CO or CO2. Consequently, the animals must be kept under regular supervision, and the temperature must be checked at regular intervals.

Radiant heaters develop their infrared thermal radiation via a ceramic body that is heated to 600-800 °C (red-hot) by the biogas flame.

The heating capacity of the radiant heater is defined by multiplying the gas flow by its net calorific value (E = Q x n.v.c.), since 95% of the biogas' energy content is converted to heat. Small-heater outputs range from 1.5 to 10 kW thermal power.

Commercial-type heaters are designed for operating on butane, propane and natural gas at a supply pressure of between 30 and 80 mbar. Since the primary air supply is. factory-set, converting a heater for biogas fueling normally consists of replacing the injector; experience shows that biogas heaters rarely work satisfactorily because the biogas has a low net calorific value and the gas supply pressure is below 20 mbar, in which case the ceramic panel is not adequately heated, i.e. the flame does not reach the entire surface, and the heater is very susceptible to draft.

Biogas-fueled radiant heaters should always be equipped with a safety pilot, and an air filter is required for sustained operation in dusty barns.

Table 5.21: Artificial brooding requirements, exemplified for a chick incubator (Source: Wesenberg 1985)

Incubation heat

37.8 °C at the beginning, declining to 30.0 °C at the end of the incubation period. The temperature should be kept as constant as possible. Any temperature in excess of 39 °c can damage the eggs.

Hatching time:

approximately 21 days

Relative humidity:

60-90 %

Ventilation:

A steady supply of fresh air (but not draft) is required to keep the CO2 content below 0.8 %.

Turning the eggs:

Incubating eggs must be turned as often as 8 times a day to keep the chicks from sticking to the inside of the shell.

Barren eggs:

Unfertilized eggs and eggs containing dead chicks must be removed (danger of infection). The eggs should be candletested once per week to ensure timely detection.



Fig. 5.34: Schematic drawing of an incubator. 1 Incubating chamber, 2 Removable tray, 3 Cover/ venting lid, 4 Heating element, 41 Heating coil, 42 Burner, 43 Gas pipe, 5 Water filler neck and expansion tank, 6 Vent valve, 7 Warming element (plastic hose). Biogas consumption rate: 30-50 1/h (Source: Wesenberg 1985)

Incubators

Incubators are supposed to imitate and maintain optimal conditions for hatching eggs. They are used to increase brooding efficiency. Indirectly warm-water-heated planar-type incubators in which a biogas burner heats water in a heating element for circulation through the incubating chamber are suitable for operating on biogas. The temperature is controlled by ether-cell-regulated vents (cf. fig. 5.34).

Refrigerators

Absorption-type refrigerating machines operating on ammonia and water and equipped for automatic thermosiphon circulation can be fueled with biogas.
Since biogas is only the refrigerator's external source of heat, just the burner itself has to be modified. Whenever a refrigerator is converted for operating on biogas, care must be taken to ensure that all safety features (safety pilot) function properly; remote ignition via a piezoelectric element substantially increases the ease of operation.

Table 5.22: Technical data of absorption refrigerators (Source: OEKOTOP)

Heating medium

gas, kerosene, electricity

Max. ambient temperature

40ºC

Heating temperature

100-150 °C

Cooling temperature


- refrigerator

5 - 10 °C

- freezer

down to approx. -12 °C

Efficiency

1.5 - 4.0% of the thermal input

Gas consumption

a) calculable via the desired refrigeration capacity


b) conversion of factory data via power input

Energy-

1-4 W/l useful volume

consumption indices

0.3-0.81 biogas/l useful volume X h


5.5.4 Biogas-fueled engines

Basic considerations

The following types of engines are, in principle, well-suited for operating on biogas:

- Four-stroke diesel engines: A diesel engine draws in air and compresses it at a ratio of 17: 1 under a pressure of approximately 30-40 bar and a temperature of about 700 °C. The injected fuel charge ignites itself. Power output is controlled by varying the injected amount of fuel, i.e. the air intake remains constant (so-called mixture control).

- Four-stroke spark-ignition engines: A spark-ignition engine (gasoline engine) draws in a mixture of fuel (gasoline or gas) and the required amount of combustion air. The charge is ignited by a spark plug at a comparably low compression ratio of between 8: 1 and 12: 1. Power control is effected by varying the mixture intake via a throttle (so-called charge control).

Four-stroke diesel and spark-ignition engines are available in standard versions with power ratings ranging from 1 kW to more than 100 kW. Less suitable for biogas fueling are:

- loop-scavenging 2-stroke engines in which lubrication is achieved by adding oil to the liquid fuel, and

- large, slow-running (less than 1000 r.p.m.) engines that are not built in large series, since they are accordingly expensive and require complicated control equipment.

Biogas engines are generally suitable for powering vehicles like tractors and light-duty trucks (pickups, vans). The fuel is contained in 200-bar steel cylinders (e.g. welding-gas cylinders). The technical, safety, instrumentational and energetic cost of gas compression, storage and filing is substantial enough to hinder large-scale application. Consequently, only stationary engines are discussed below.

Essential terms and definitions

Knowledge of the following terms pertaining to internal combustion engines is requisite to understanding the context:

Piston displacement is the volume (cm³, l) displaced by a piston in a cylinder in a single stroke, i.e. between the bottom and . top dead-canter positions (BDC and TDC, respectively). The total cylinder capacity (Vtot) comprises the swept volume (Vs) and the compression volume (Vc), i.e. Vtot = Vs+Vc.

The compression ratio (E) is the ratio of the maximum to the minimum volume of the space enclosed by the piston, i.e. prior to compression (Vtot) as compared to the end of the compression stroke (Vc). The compression ratio can be used to calculate the pressure and temperature of the compressed fuel mixture (E = Vtot/Vc).

The efficiency (rl = Pc/Pf) is the ratio between the power applied to the crankshaft (Pc) and the amount of energy introduced with the fuel (Pf = V x n.c.v.).

Ignition and combustion: The firing point (diesel: flash point; spark-ignition engine: ignition point) is timed to ensure that the peak pressure is reached just after the piston passes top dead center (approx. 10° - 15° crankshaft angle). Any deviation from the optimal fiash/ignition point leads to a loss of power and efficiency; in extreme cases, the engine may even suffer damage. The flash/ignition point is chosen on the basis of the time history of combustion, i.e. the rate of combustion, and depends on the compression pressure, type of fuel, combustion-air/ fuel ratio and the engine speed. The ignition timing (combustion) must be such that the air/fuel mixture is fully combusted at the end of the combustion cycle, i.e. when the exhaust valve opens, since part of the fuel's energy content would otherwise be wasted.

Air/Fuel-ratio and control: Proper combustion requires a fuel-dependent stoichiometric air/fuel-ratio (af-ratio). As a rule, the quality of combustion is maximized by increasing the air fraction, as expressed by the air-ratio coefficient (d = actual air volume/stoichiometric air volume).

For gasoline and gas-fueled engines, the optimal air/fuel ratio is situated somewhere within the range d = 0.8 - 1.3, with maximum power output at 0.9 and maximum efficiency (and clean exhaust) at 1.1. The power output is controlled by varying the mixture intake and, hence, the cylinder's volumetric efficient and final pressure, via the throttle. Diesel engines require an air-ratio of d = 1.3 at full load and 4 - 6 at low load, i.e. fuel intake is reduced, while the air intake remains constant.

Converting diesel engines

Diesel engines are designed for continuous operation (10 000 or more operating hours). Basically, they are well-suited for conversion to biogas according to either of two methods:

The dual-fuel approach

Except for the addition of a gas/air mixing chamber on the intake manifold (if need be, the air filter can be used as a mixing chamber), the diesel engine remains extensively unmodified. The injected diesel fuel still ignites itself, while the amount injected is automatically reduced by the speed governor, depending on how much biogas is introduced into the mixing chamber. The biogas supply is controlled by hand. The maximum biogas intake must be kept below the point at which the engine would begin to stutter. If that happens, the governor is getting too much biogas and has therefore turned down the diesel intake so far that ignition is no longer steady. Normally, 15 - 20% diesel is sufficiency, meaning that as much as 80% of the diesel fuel can be replaced by biogas. Any lower share of biogas can also be used, of course, since the governer automatically compensates with more diesel.

As a rule, dual-fuel diesels perform just as well as a comparable engine operating on pure diesel.

As in normal diesel operation, the speed is controlled by an accelerator lever, and load control is normally effected by hand, i.e. by adjusting the biogas valve (keeping in mind the maximum acceptable biogas intake level). In case of frequent power changes joined with steady speed, the biogas fraction should be reduced somewhat to let the governer decrease the diesel intake without transgressing the minimum amount. Thus, the speed is kept constant, even in case of power cycling. Important: No diesel engine should be subjected to air-side control.

While special T-pieces or mixing chambers with 0.5 to 1.0 times the engine displacement can serve as the diesel/biogas mixing chamber, at which a true mixing chamber offers the advantage of more thorough mixing.

Conversion according to the dual-fuel method is evaluated as follows

- a quick & easy do-it-yourself technique
- will accommodate an unsteady supply of biogas
- well-suited for steady operation, since a single manual adjustment will suffice
- requires a minimum share of diesel to ensure ignition.

Conversion to spark ignition (Otto cycle)

involves the following permanent alterations to the engine:

- removing the fuel-injection pump and nozzle
- adding an ignition distributor and an ignition coil with power supply (battery or dynamo)
- installing spark plugs in place of the injection nozzles
- adding a gas mixing valve or carburetor
- adding a throttle control device
- reducing the compression ratio to E = 11-12
- observing the fact that, as a rule, engines with a precombustion or swirl chamber are not suitable for such conversion.

Converting a diesel engine to a biogas-fueled spark-ignition engine is very expensive and complicated - so much so, that only preconverted engines of that type should be procured.

Converting spark-ignition engines

Converting a spark-ignition engine for biogas fueling requires replacement of the gasoline carburetor with a mixing valve (pressure-controlled venturi type or with throttle). The spark-ignition principle is retained, but should be advanced as necessary to account for slower combustion (approx. 5°-10° crankshaft angle) and to avoid overheating of the exhaust valve while precluding loss of energy due to still-combustible exhaust gases. The engine speed should be limited to 3000 r.p.m. for the same reason. As in the case of diesel-engine conversion, a simple mixing chamber should normally suffice for continuous operation at a steady speed. In addition, however, the mixing chamber should be equipped with a hand-operated air-side control valve for use in adjusting the air/fuel ratio (opt. d = 1.1).

Table 5.23: Engine-conversion requirements for various duty and control modes (Source: Mitzlaff 1986)

Duty mode

Control mode

Conversion mode

Speed: constant power: constant e.g. for a pump with constant head and constant delivery

Diesel or spark- ignition engine: fixed manual adjustment, no readjustment necessary under normal circumstances

addition of a simple, manually adjusted mixing chamber

Speed: constant power: variable e.g. for a constant-frequency subject to varying power; or for a pump with constant head and varying delivery volume

Automatic speed control: Spark-ignition: electronic governor controls the throttle Diesel: fixed biogas fraction, with speed control via diesel intake governor

Spark-ignition: carburetor or gas mixing valve with throttle; elec tronic control Diesel: Regulator and hand-adjusted mixing chamber

Speed: variable power: variable e.g. for powering various types of machines

Spark-ignition: by hand (if varying speed is acceptable) or electric with setpoint control Diesel: by hand via accelerator Iever

Spark-ignition: electronic with set point control, gas mixing valve or carburetor with throttle, plus regu lator Diesel: simple, hand-adjusted mixing chamber



Fig. 5.35: Various gas mixers for spark-ignition and diesel engines. 1 Air intake, 2 Air filter, 3 Biogas supply pipe, 4 Biogas control valve, 5 Mixing chamber (0.5 - 1 X piston displacement) 6 Throttle, 7 Mixing valve (Source: OEKOTOP)

Converting a spark-ignition engine results in a loss of performance amounting to as much as 3070. While partial compensation can be achieved by raising the compression ratio to E = 11-12, such a measure also in,creases the mechanical and thermal load on the engine.
Spark-ignition engines that are not expressly marketed as suitable for running on gas or unleaded gasoline may suffer added wear & tear due to the absence of lead lubrication.

The speed control of converted spark-ignition engines is effected by way of a hand-operated throttle. Automatic speed control for different load conditions requires the addition of an electronic control device for the throttle.

The conversion of spark-ignition engines is evaluated as follows:

- Gasoline engines are readily available in the form of vehicle motors, but their useful life amounts to a mere 3000 - 4000 operating hours.

- The conversion effort essentially consists of adding a (well-tuned) gas mixer.

- Gasoline engines are not as durable as diesel engines.

Engine selection and operation

Speed

Since biogas burns relatively slowly, biogas-fueled engines should be operated at
- 1300-2000 r.p.m. (diesel)
- 1500-3000 r.p.m. (spark-ignition)

The standard speeds for such engines are 1500 and 3000 r.p.m. (50 Hz) or 1800/3600 r.p.m. (60 Hz) because of connecting a generator. For direct-power applications, i.e. a V-belt drive, the transmission ratio should ensure that the engine operates within its best efficiency range (= lowest fuel consumption) under normal-power conditions.

(f engine-end pulley speed of machine)/(f machine-end pulley)= (speed of machine)/(speed of engine)

Consumption

Depending on the gas composition, barometric pressure and type of engine, the specific consumption will amount to 0.5-0.8 m³/ kWh, i.e. a 10-kW engine will use 5-8 m³ biogas per hour. In a dual-fuel setup, the biogas consumption rate can be reduced by lowering the biogas fraction.


Fig. 5.36: Consumption of diesel and biogas by a 10-kW engine (1 cyl., 1000 ccm), 1300 m above sea level, running at 1500 r.p.m. 1 Biogas consumption in dual-fuel operation, 2 Diesel consumption in pure diesel operation, 3 Diesel consumption in dual-fuel operation, 4 Diesel saving, 5 Efficiency in diesel operation, 6 Efficiency in dual-fuel operation (Source: Mitzlaff 1986)

Maintenance and useful life

In contact with water, the H2S content of biogas promotes corrosion. Consequently, adherence to the prescribed oil-change intervals is very important (after each 100 operating hours or so for vehicle spark-ignition engines). Dual-fuel engines should be started on pure diesel, with biogas being added gradually after about 2 minutes. For shutdown, the biogas fraction should be gradually reduced prior to stopping the engine. Any engine that has not been in operation for a considerable length of time should first be flushed out with scavenge oil (50% motor oil, 50% diesel oil) and filled with fresh oil. As long as extreme operating conditions are avoided, the engine can expected to achieve its normal useful life.

Exhaust-heat utilization

Internal-combustion engines have efficiency levels of 25 - 30% (gasoline engine) and 33 - 38% (diesel engine). A higher overall efficiency can be achieved by exploiting the heat content of the cooling water and exhaust, e.g. by:

- an exhaust heat exchanger (danger of H2O-corrosion if the exhaust gas cools down to 150 °C or less)

- coolant heat exchanger (at coolant temperatures of 60 - 70 °C).


Fig. 5.37: Energy shares of an internal-combustion engine. 1 Energy input, 2 Dissipated energy (radiant heat and exhaust), 3 Useful exhaust energy, 4 Thermal energy in cooling water, 5 Mechanical power applied to crankshaft (Source: Mitzlaff 1986)

The recovered heat can be used for:

- heating utility water
- drying agricultural products
- space heating.

However, the requisite equipment/control effort makes heat recovery uneconomical except for large heavy-duty engines.

Motor-generators

The most frequent use for biogas-fueled engines is the generation of electricity. Suitable components include:

- asynchronous generators for system interconnection, i.e. the generator can only be operated in connection with a central power network. If the network breaks down, the generator cannot stay in operation. System control and network adaptation are relatively uncomplicated.

- asynchronous generators for insular networks, i.e. an electronic control system on the generator stabilizes a constant power network.

Converting one type of generator to the other is very intricate and involves a complicated electronic control arrangement.

In selecting a particular type of motor generator, one must give due consideration to the various operating conditions and network requirements (including the legal aspects of power feed-in).

Checklist for choosing a suitable engine

1. Define the energy requirement and speed of the machine to be powered;

2. Compare the biogas demand with the given storage capacity; if a shortage is possible, opt for the dual-fuel approach;

3. Select an engine with performance characteristics that are sure to provide the required power output in sustained operation in the optimal duty range:

- diesel engines Pengine = Pmachine/0.8
- gasoline engines Pengine = Pmachine/0.G

This accounts for the fact that the continuous-duty power output is less than the nominal output. On the other hand, choosing an overly powerful engine would make the specific consumption unnecessarily high. Careful planning is very important in any project involving the use of biogas in engines; experienced technicians are needed to make the engine connections; and access to maintenance and repair services is advisable. Both the biogas plant itself and the engine require protection in the form of a low-pressure cutout that shuts down the latter if the gasholder is empty. Chapter 10.5 lists some recommended types of biogas engines and supplier addresses.